Internet Engineering Task Force (IETF) T. Paila
Request for Comments: 6726 Nokia
Obsoletes: 3926 R. Walsh
Category: Standards Track Nokia/TUT
ISSN: 2070-1721 M. Luby
Qualcomm Technologies, Inc.
V. Roca
INRIA
R. Lehtonen
TeliaSonera
November 2012
FLUTE - File Delivery over Unidirectional Transport
Abstract
This document defines File Delivery over Unidirectional Transport
(FLUTE), a protocol for the unidirectional delivery of files over the
Internet, which is particularly suited to multicast networks. The
specification builds on Asynchronous Layered Coding, the base
protocol designed for massively scalable multicast distribution.
This document obsoletes RFC 3926.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6726.
Paila, et al. Standards Track [Page 1]

RFC 6726 FLUTE November 2012
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction ....................................................31.1. Applicability Statement ....................................51.1.1. The Target Application Space ........................51.1.2. The Target Scale ....................................51.1.3. Intended Environments ...............................51.1.4. Weaknesses ..........................................62. Conventions Used in This Document ...............................63. File Delivery ...................................................73.1. File Delivery Session ......................................83.2. File Delivery Table .......................................103.3. Dynamics of FDT Instances within a File Delivery Session ..12
3.4. Structure of FDT Instance Packets .........................153.4.1. Format of FDT Instance Header ......................163.4.2. Syntax of FDT Instance .............................173.4.3. Content Encoding of FDT Instance ...................213.5. Multiplexing of Files within a File Delivery Session ......224. Channels, Congestion Control, and Timing .......................235. Delivering FEC Object Transmission Information .................246. Describing File Delivery Sessions ..............................26Paila, et al. Standards Track [Page 2]

RFC 6726 FLUTE November 20127. Security Considerations ........................................277.1. Problem Statement .........................................277.2. Attacks against the Data Flow .............................287.2.1. Access to Confidential Files .......................287.2.2. File Corruption ....................................28
7.3. Attacks against the Session Control Parameters and
Associated Building Blocks ................................307.3.1. Attacks against the Session Description ............307.3.2. Attacks against the FDT Instances ..................317.3.3. Attacks against the ALC/LCT Parameters .............317.3.4. Attacks against the Associated Building Blocks .....327.4. Other Security Considerations .............................327.5. Minimum Security Recommendations ..........................338. IANA Considerations ............................................348.1. Registration of the FDT Instance XML Namespace ............348.2. Registration of the FDT Instance XML Schema ...............348.3. Registration of the application/fdt+xml Media Type ........35
8.4. Creation of the FLUTE Content Encoding Algorithms
Registry ..................................................368.5. Registration of LCT Header Extension Types ................369. Acknowledgments ................................................3610. Contributors ..................................................3711. Change Log ....................................................3711.1. RFC 3926 to This Document ................................3712. References ....................................................4012.1. Normative References .....................................4012.2. Informative References ...................................41Appendix A. Receiver Operation (Informative) ......................44Appendix B. Example of FDT Instance (Informative) .................451. Introduction
This document defines FLUTE version 2, a protocol for unidirectional
delivery of files over the Internet. This specification is not
backwards compatible with the previous experimental version defined
in [RFC3926] (see Section 11 for details). The specification builds
on Asynchronous Layered Coding (ALC), version 1 [RFC5775], the base
protocol designed for massively scalable multicast distribution. ALC
defines transport of arbitrary binary objects. For file delivery
applications, mere transport of objects is not enough, however. The
end systems need to know what the objects actually represent. This
document specifies a technique called FLUTE -- a mechanism for
signaling and mapping the properties of files to concepts of ALC in a
way that allows receivers to assign those parameters for received
objects. Consequently, throughout this document the term 'file'
relates to an 'object' as discussed in ALC. Although this
Paila, et al. Standards Track [Page 3]

RFC 6726 FLUTE November 2012
specification frequently makes use of multicast addressing as an
example, the techniques are similarly applicable for use with unicast
addressing.
This document defines a specific transport application of ALC, adding
the following specifications:
- Definition of a file delivery session built on top of ALC,
including transport details and timing constraints.
- In-band signaling of the transport parameters of the ALC session.
- In-band signaling of the properties of delivered files.
- Details associated with the multiplexing of multiple files within
a session.
This specification is structured as follows. Section 3 begins by
defining the concept of the file delivery session. Following that,
it introduces the File Delivery Table, which forms the core part of
this specification. Further, it discusses multiplexing issues of
transmission objects within a file delivery session. Section 4
describes the use of congestion control and channels with FLUTE.
Section 5 defines how the Forward Error Correction (FEC) Object
Transmission Information is to be delivered within a file delivery
session. Section 6 defines the required parameters for describing
file delivery sessions in a general case. Section 7 outlines
security considerations regarding file delivery with FLUTE. Last,
there are two informative appendices. Appendix A describes an
envisioned receiver operation for the receiver of the file delivery
session. Readers who want to see a simple example of FLUTE in
operation should refer to Appendix A right away. Appendix B gives an
example of a File Delivery Table.
This specification contains part of the definitions necessary to
fully specify a Reliable Multicast Transport (RMT) protocol in
accordance with [RFC2357].
This document obsoletes [RFC3926], which contained a previous version
of this specification and was published in the "Experimental"
category. This Proposed Standard specification is thus based on
[RFC3926] and has been updated according to accumulated experience
and growing protocol maturity since the publication of [RFC3926].
Said experience applies both to this specification itself and to
congestion control strategies related to the use of this
specification.
Paila, et al. Standards Track [Page 4]

RFC 6726 FLUTE November 2012
The differences between [RFC3926] and this document are listed in
Section 11.
This document updates ALC [RFC5775] and Layered Coding Transport
(LCT) [RFC5651] in the sense that it defines two new header
extensions, EXT_FDT and EXT_CENC.
1.1. Applicability Statement1.1.1. The Target Application Space
FLUTE is applicable to the delivery of large and small files to many
hosts, using delivery sessions of several seconds or more. For
instance, FLUTE could be used for the delivery of large software
updates to many hosts simultaneously. It could also be used for
continuous, but segmented, data such as time-lined text for
subtitling -- potentially leveraging its layering inheritance from
ALC and LCT to scale the richness of the session to the congestion
status of the network. It is also suitable for the basic transport
of metadata, for example, Session Description Protocol (SDP)
[RFC4566] files that enable user applications to access multimedia
sessions.
1.1.2. The Target Scale
Massive scalability is a primary design goal for FLUTE. IP multicast
is inherently massively scalable, but the best-effort service that it
provides does not provide session management functionality,
congestion control, or reliability. FLUTE provides all of this by
using ALC and IP multicast without sacrificing any of the inherent
scalability of IP multicast.
1.1.3. Intended Environments
All of the environmental requirements and considerations that apply
to the RMT building blocks used by FLUTE shall also apply to FLUTE.
These are the ALC protocol instantiation [RFC5775], the LCT building
block [RFC5651], and the FEC building block [RFC5052].
FLUTE can be used with both multicast and unicast delivery, but its
primary application is for unidirectional multicast file delivery.
FLUTE requires connectivity between a sender and receivers but does
not require connectivity from receivers to a sender. Because of its
low expectations, FLUTE works with most types of networks, including
LANs, WANs, Intranets, the Internet, asymmetric networks, wireless
networks, and satellite networks.
Paila, et al. Standards Track [Page 5]

RFC 6726 FLUTE November 2012
FLUTE is compatible with both IPv4 and IPv6, as no part of the packet
is IP version specific. FLUTE works with both multicast models:
Any-Source Multicast (ASM) [RFC1112] and Source-Specific Multicast
(SSM) [PAPER.SSM].
FLUTE is applicable for both shared networks, such as the Internet,
with a suitable congestion control building block; and provisioned/
controlled networks, such as wireless broadcast radio systems, with a
traffic-shaping building block.
1.1.4. Weaknesses
FLUTE congestion control protocols depend on the ability of a
receiver to change multicast subscriptions between multicast groups
supporting different rates and/or layered codings. If the network
does not support this, then the FLUTE congestion control protocols
may not be amenable to such a network.
FLUTE can also be used for point-to-point (unicast) communications.
At a minimum, implementations of ALC MUST support the Wave and
Equation Based Rate Control (WEBRC) [RFC3738] multiple-rate
congestion control scheme [RFC5775]. However, since WEBRC has been
designed for massively scalable multicast flows, it is not clear how
appropriate it is to the particular case of unicast flows. Using a
separate point-to-point congestion control scheme is another
alternative. How to do that is outside the scope of the present
document.
FLUTE provides reliability using the FEC building block. This will
reduce the error rate as seen by applications. However, FLUTE does
not provide a method for senders to verify the reception success of
receivers, and the specification of such a method is outside the
scope of this document.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The terms "object" and "transmission object" are consistent with the
definitions in ALC [RFC5775] and LCT [RFC5651]. The terms "file" and
"source object" are pseudonyms for "object".
Paila, et al. Standards Track [Page 6]

RFC 6726 FLUTE November 20123. File Delivery
Asynchronous Layered Coding [RFC5775] is a protocol designed for
delivery of arbitrary binary objects. It is especially suitable for
massively scalable, unidirectional multicast distribution. ALC
provides the basic transport for FLUTE, and thus FLUTE inherits the
requirements of ALC.
This specification is designed for the delivery of files. The core
of this specification is to define how the properties of the files
are carried in-band together with the delivered files.
As an example, let us consider a 5200-byte file referred to by
"http://www.example.com/docs/file.txt". Using the example, the
following properties describe the properties that need to be conveyed
by the file delivery protocol.
* Identifier of the file, expressed as a URI [RFC3986]. The
identifier MAY provide a location for the file. In the above
example: "http://www.example.com/docs/file.txt".
* File name (usually, this can be concluded from the URI). In the
above example: "file.txt".
* File type, expressed as Internet Media Types (often referred to as
"Media Types"). In the above example: "text/plain".
* File size, expressed in octets. In the above example: "5200". If
the file is content encoded, then this is the file size before
content encoding.
* Content encoding of the file, within transport. In the above
example, the file could be encoded using ZLIB [RFC1950]. In this
case, the size of the transmission object carrying the file would
probably differ from the file size. The transmission object size
is delivered to receivers as part of the FLUTE protocol.
* Security properties of the file, such as digital signatures,
message digests, etc. For example, one could use S/MIME [RFC5751]
as the content encoding type for files with this authentication
wrapper, and one could use XML Digital Signatures (XML-DSIG)
[RFC3275] to digitally sign the file. XML-DSIG can also be used
to provide tamper prevention, e.g., in the Content-Location field.
Content encoding is applied to file data before FEC protection.
Paila, et al. Standards Track [Page 7]

RFC 6726 FLUTE November 2012
For each unique file, FLUTE encodes the attributes listed above and
other attributes as children of an XML file element. A table of XML
file elements is transmitted as a special file called a 'File
Delivery Table' (FDT), which is further described in the next
subsection and in Section 3.2.
3.1. File Delivery Session
ALC is a protocol instantiation of the Layered Coding Transport (LCT)
building block [RFC5651]. Thus, ALC inherits the session concept of
LCT. In this document, we will use the concept of the ALC/LCT
session to collectively denote the interchangeable terms "ALC
session" and "LCT session".
An ALC/LCT session consists of a set of logically grouped ALC/LCT
channels associated with a single sender sending ALC/LCT packets for
one or more objects. An ALC/LCT channel is defined by the
combination of a sender and an address associated with the channel by
the sender. A receiver joins a channel to start receiving the data
packets sent to the channel by the sender, and a receiver leaves a
channel to stop receiving data packets from the channel.
One of the fields carried in the ALC/LCT header is the Transport
Session Identifier (TSI), an integer carried in a field of size 16,
32, or 48 bits (note that the TSI may be carried by other means, in
which case it is absent from the LCT header [RFC5651]). The (source
IP address, TSI) pair uniquely identifies a session. Note that the
TSI is scoped by the IP address, so the same TSI may be used by
several source IP addresses at once. Thus, the receiver uses the
(source IP address, TSI) pair from each packet to uniquely identify
the session sending each packet. When a session carries multiple
objects, the Transmission Object Identifier (TOI) field within the
ALC/LCT header names the object used to generate each packet. Note
that each object is associated with a unique TOI within the scope of
a session.
A FLUTE session consistent with this specification MUST use FLUTE
version 2 as specified in this document. Thus, all sessions
consistent with this specification MUST set the FLUTE version to 2.
The FLUTE version is carried within the EXT_FDT Header Extension
(defined in Section 3.4.1) in the ALC/LCT layer. A FLUTE session
consistent with this specification MUST use ALC version 1 as
specified in [RFC5775], and LCT version 1 as specified in [RFC5651].
If multiple FLUTE sessions are sent to a channel, then receivers MUST
determine the FLUTE protocol version, based on version fields and the
(source IP address, TSI) pair carried in the ALC/LCT header of the
packet. Note that when a receiver first begins receiving packets, it
Paila, et al. Standards Track [Page 8]

RFC 6726 FLUTE November 2012
might not know the FLUTE protocol version, as not every LCT packet
carries the EXT_FDT header (containing the FLUTE protocol version).
A new receiver MAY keep an open binding in the LCT protocol layer
between the TSI and the FLUTE protocol version, until the EXT_FDT
header arrives. Alternatively, a new receiver MAY discover a binding
between TSI and FLUTE protocol version via a session discovery
protocol that is out of scope of this document.
If the sender's IP address is not accessible to receivers, then
packets that can be received by receivers contain an intermediate IP
address. In this case, the TSI is scoped by this intermediate IP
address of the sender for the duration of the session. As an
example, the sender may be behind a Network Address Translation (NAT)
device that temporarily assigns an IP address for the sender. In
this case, the TSI is scoped by the intermediate IP address assigned
by the NAT. As another example, the sender may send its original
packets using IPv6, but some portions of the network may not be IPv6
capable. Thus, there may be an IPv6-to-IPv4 translator that changes
the IP address of the packets to a different IPv4 address. In this
case, receivers in the IPv4 portion of the network will receive
packets containing the IPv4 address, and thus the TSI for them is
scoped by the IPv4 address. How the IP address of the sender to be
used to scope the session by receivers is delivered to receivers,
whether it is the sender's IP address or an intermediate IP address,
is outside the scope of this document.
When FLUTE is used for file delivery over ALC, the ALC/LCT session is
called a file delivery session, and the ALC/LCT concept of 'object'
denotes either a 'file' or a 'File Delivery Table Instance'
(Section 3.2).
Additionally, the following rules apply:
* The TOI field MUST be included in ALC packets sent within a FLUTE
session, with the exception that ALC packets sent in a FLUTE
session with the Close Session (A) flag set to 1 (signaling the
end of the session) and that contain no payload (carrying no
information for any file or FDT) SHALL NOT carry the TOI. See
Section 5.1 of [RFC5651] for the LCT definition of the Close
Session flag, and see Section 4.2 of [RFC5775] for an example of
the use of a TOI within an ALC packet.
* The TOI value '0' is reserved for the delivery of File Delivery
Table Instances. Each non-expired File Delivery Table Instance is
uniquely identified by an FDT Instance ID within the EXT_FDT
header defined in Section 3.4.1.
Paila, et al. Standards Track [Page 9]

RFC 6726 FLUTE November 2012
* Each file in a file delivery session MUST be associated with a TOI
(>0) in the scope of that session.
* Information carried in the headers and the payload of a packet is
scoped by the source IP address and the TSI. Information
particular to the object carried in the headers and the payload of
a packet is further scoped by the TOI for file objects, and is
further scoped by both the TOI and the FDT Instance ID for FDT
Instance objects.
3.2. File Delivery Table
The File Delivery Table (FDT) provides a means to describe various
attributes associated with files that are to be delivered within the
file delivery session. The following lists are examples of such
attributes and are not intended to be mutually exclusive or
exhaustive.
Attributes related to the delivery of a file:
- TOI value that represents the file
- FEC Object Transmission Information (including the FEC Encoding ID
and, if relevant, the FEC Instance ID)
- Size of the transmission object carrying the file
- Aggregate rate of sending packets to all channels
Attributes related to the file itself:
- Name, Identification, and Location of file (specified by the URI)
- Media type of file
- Size of file
- Encoding of file
- Message digest of file
Some of these attributes MUST be included in the file description
entry for a file; others are optional, as defined in Section 3.4.2.
Logically, the FDT is a set of file description entries for files to
be delivered in the session. Each file description entry MUST
include the TOI for the file that it describes and the URI
identifying the file. The TOI carried in each file description entry
Paila, et al. Standards Track [Page 10]

RFC 6726 FLUTE November 2012
is how FLUTE names the ALC/LCT data packets used for delivery of the
file. Each file description entry may also contain one or more
descriptors that map the above-mentioned attributes to the file.
Each file delivery session MUST have an FDT that is local to the
given session. The FDT MUST provide a file description entry mapped
to a TOI for each file appearing within the session. An object that
is delivered within the ALC session, but not described in the FDT,
other than the FDT itself, is not considered a 'file' belonging to
the file delivery session. This object received with an unmapped TOI
(non-zero TOI that is not resolved by the FDT) SHOULD in general be
ignored by a FLUTE receiver. The details of how to do that are out
of scope of this specification.
Note that a client that joins an active file delivery session MAY
receive data packets for a TOI > 0 before receiving any FDT Instance
(see Section 3.3 for recommendations on how to limit the probability
that this situation will occur). Even if the TOI is not mapped to
any file description entry, this is hopefully a transient situation.
When this happens, system performance might be improved by caching
such packets within a reasonable time window and storage size. Such
optimizations are use-case and implementation specific, and further
details are beyond the scope of this document.
Within the file delivery session, the FDT is delivered as FDT
Instances. An FDT Instance contains one or more file description
entries of the FDT. Any FDT Instance can be equal to, be a subset
of, be a superset of, overlap with, or complement any other FDT
Instance. A certain FDT Instance may be repeated multiple times
during a session, even after subsequent FDT Instances (with higher
FDT Instance ID numbers) have been transmitted. Each FDT Instance
contains at least a single file description entry and at most the
exhaustive set of file description entries of the files being
delivered in the file delivery session.
A receiver of the file delivery session keeps an FDT database for
received file description entries. The receiver maintains the
database, for example, upon reception of FDT Instances. Thus, at any
given time the contents of the FDT database represent the receiver's
current view of the FDT of the file delivery session. Since each
receiver behaves independently of other receivers, it SHOULD NOT be
assumed that the contents of the FDT database are the same for all
the receivers of a given file delivery session.
Since the FDT database is an abstract concept, the structure and the
maintenance of the FDT database are left to individual
implementations and are thus out of scope of this specification.
Paila, et al. Standards Track [Page 11]

RFC 6726 FLUTE November 20123.3. Dynamics of FDT Instances within a File Delivery Session
The following rules define the dynamics of the FDT Instances within a
file delivery session:
* For every file delivered within a file delivery session, there
MUST be a file description entry included in at least one FDT
Instance sent within the session. A file description entry
contains at a minimum the mapping between the TOI and the URI.
* An FDT Instance MAY appear in any part of the file delivery
session, and packets for an FDT Instance MAY be interleaved with
packets for other files or other FDT Instances within a session.
* The TOI value of '0' MUST be reserved for delivery of FDT
Instances. The use of other TOI values (i.e., an integer > 0) for
FDT Instances is outside the scope of this specification.
* The FDT Instance is identified by the use of a new fixed-length
LCT Header Extension, EXT_FDT (defined later in this section).
Each non-expired FDT Instance is uniquely identified within the
file delivery session by its FDT Instance ID, carried by the
EXT_FDT Header Extension. Any ALC/LCT packet carrying an FDT
Instance MUST include EXT_FDT.
* It is RECOMMENDED that an FDT Instance that contains the file
description entry for a file be sent at least once before sending
the described file within a file delivery session. This
recommendation is intended to minimize the amount of file data
that may be received by receivers in advance of the FDT Instance
containing the entry for a file (such data must either be
speculatively buffered or discarded). Note that this possibility
cannot be completely eliminated, since the first transmission of
FDT data might be lost.
* Within a file delivery session, any TOI > 0 MAY be described more
than once. For example, a previous FDT Instance 0 describes a TOI
of value '3'. Now, subsequent FDT Instances can either keep TOI
'3' unmodified in the table, not include it, or augment the
description. However, subsequent FDT Instances MUST NOT change
the parameters already described for a specific TOI.
* An FDT Instance is valid until its expiration time. The
expiration time is expressed within the FDT Instance payload as a
UTF-8 decimal representation of a 32-bit unsigned integer. The
value of this integer represents the 32 most significant bits of a
64-bit Network Time Protocol (NTP) [RFC5905] time value. These
32 bits provide an unsigned integer representing the time in
Paila, et al. Standards Track [Page 12]

RFC 6726 FLUTE November 2012
seconds relative to 0 hours 1 January 1900 in the case of the
prime epoch (era 0) [RFC5905]. The handling of time wraparound
(to happen in 2036) requires that the associated epoch be
considered. In any case, both a sender and a receiver easily
determine to which (136-year) epoch the FDT Instance expiration
time value pertains by choosing the epoch for which the expiration
time is closest in time to the current time.
Here is an example. Let us imagine that a new FLUTE session is
started on February 7th, 2036, 0h, i.e., at NTP time
4,294,944,000, a few hours before the end of epoch 0. In order to
define an FDT Instance valid for the next 48 hours, The FLUTE
sender sets an expiry time of 149,504. This FDT Instance will
expire exactly on February 9th, 2036, 0h. A client that receives
this FDT Instance on the 7th, 0h, just after it has been sent,
immediately understands that this value corresponds to epoch 1. A
client that joins the session on February 8th, 0h, i.e., at NTP
time 63,104, epoch 1, immediately understands that the 149,504 NTP
timestamp corresponds to epoch 1.
* The space of FDT Instance IDs is limited by the associated field
size (i.e., 20 bits) in the EXT_FDT Header Extension
(Section 3.4.1). Therefore, senders should take care to always
have a large enough supply of available FDT Instance IDs when
specifying FDT expiration times.
* The receiver MUST NOT use a received FDT Instance to interpret
packets received beyond the expiration time of the FDT Instance.
* A sender MUST use an expiration time in the future upon creation
of an FDT Instance relative to its Sender Current Time (SCT).
* Any FEC Encoding ID MAY be used for the sending of FDT Instances.
The default is to use the Compact No-Code FEC Encoding ID 0
[RFC5445] for the sending of FDT Instances. (Note that since FEC
Encoding ID 0 is the default for FLUTE, this implies that Source
Block Number and Encoding Symbol ID lengths both default to
16 bits each.)
* If the receiver does not support the FEC Scheme indicated by the
FEC Encoding ID, the receiver MUST NOT decode the associated FDT.
* It is RECOMMENDED that the mechanisms used for file attribute
delivery SHOULD achieve a delivery probability that is higher than
the file recovery probability and the file attributes SHOULD be
delivered at this higher priority before the delivery of the
associated files begins.
Paila, et al. Standards Track [Page 13]

RFC 6726 FLUTE November 2012
Generally, a receiver needs to receive an FDT Instance describing a
file before it is able to recover the file itself. In this sense,
FDT Instances are of higher priority than files. Additionally, a
FLUTE sender SHOULD assume that receivers will not receive all
packets pertaining to FDT Instances. The way FDT Instances are
transmitted has a large impact on satisfying the recommendation
above. When there is a single file transmitted in the session, one
way to satisfy the recommendation above is to repeatedly transmit on
a regular enough basis FDT Instances describing the file while the
file is being transmitted. If an FDT Instance is longer than one
packet payload in length, it is RECOMMENDED that an FEC code that
provides protection against loss be used for delivering this FDT
Instance. When there are multiple files in a session concurrently
being transmitted to receivers, the way the FDT Instances are
structured and transmitted also has a large impact. As an example, a
way to satisfy the recommendation above is to transmit an FDT
Instance that describes all files currently being transmitted, and to
transmit this FDT Instance reliably, using the same techniques as
explained for the case when there is a single file transmitted in a
session. If instead the concurrently transmitted files are described
in separate FDT Instances, another way to satisfy this recommendation
is to transmit all the relevant FDT Instances reliably, using the
same techniques as explained for the case when there is a single file
transmitted in a session.
In any case, how often the description of a file is sent in an FDT
Instance, how often an FDT Instance is sent, and how much FEC
protection is provided for an FDT Instance (if longer than one packet
payload) are dependent on the particular application and are outside
the scope of this document.
Sometimes the various attributes associated with files that are to be
delivered within the file delivery session are sent out-of-band. The
details of how this is done are out of the scope of this document.
However, it is still RECOMMENDED that any out-of-band transmission be
managed in such a way that a receiver will be able to recover the
attributes associated with a file at least as reliably as the
receiver is able to receive enough packets containing encoding
symbols to recover the file. For example, the probability of a
randomly chosen receiver being able to recover a given file can often
be estimated based on a statistical model of reception conditions,
the amount of data transmitted, and the properties of any Forward
Error Correction in use. The recommendation above suggests that
mechanisms used for file attribute delivery should achieve a higher
delivery probability than the file recovery probability. The sender
MAY also continue sending the various file attributes in-band, in
addition to the out-of-band transmission.
Paila, et al. Standards Track [Page 14]

RFC 6726 FLUTE November 20123.4. Structure of FDT Instance Packets
FDT Instances are carried in ALC packets with TOI = 0 and with an
additional REQUIRED LCT Header extension called the FDT Instance
Header. The FDT Instance Header (EXT_FDT) contains the FDT Instance
ID that uniquely identifies FDT Instances within a file delivery
session. Placement of the FDT Instance Header is the same as that of
any other LCT Header Extension. There MAY be other LCT Header
Extensions in use.
The FDT Instance is encoded for transmission, like any other object,
using an FEC Scheme (which MAY be the Compact No-Code FEC Scheme).
The LCT Header Extensions are followed by the FEC Payload ID, and
finally the Encoding Symbols for the FDT Instance, which contains one
or more file description entries. An FDT Instance MAY span several
ALC packets -- the number of ALC packets is a function of the file
attributes associated with the FDT Instance. The FDT Instance Header
is carried in each ALC packet carrying the FDT Instance. The FDT
Instance Header is identical for all ALC/LCT packets for a particular
FDT Instance.
The overall format of ALC/LCT packets carrying an FDT Instance is
depicted in Figure 1 below. All integer fields are carried in
"big-endian" or "network order" format (i.e., most significant byte
(octet) first). As defined in [RFC5775], all ALC/LCT packets are
sent using UDP.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP header |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Default LCT header (with TOI = 0) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LCT Header Extensions (EXT_FDT, EXT_FTI, etc.) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC Payload ID |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
FLUTE Payload: Encoding Symbol(s)
~ (for FDT Instance in an FDT packet) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Overall FDT Packet
Paila, et al. Standards Track [Page 15]

RFC 6726 FLUTE November 20123.4.1. Format of FDT Instance Header
The FDT Instance Header (EXT_FDT) is a new fixed-length, ALC
Protocol-Instantiation-specific LCT Header Extension [RFC5651]. The
Header Extension Type (HET) for the extension is 192. Its format is
defined below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HET = 192 | V | FDT Instance ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: EXT_FDT Format
Version of FLUTE (V), 4 bits:
This document specifies FLUTE version 2. Hence, in any ALC packet
that carries an FDT Instance and that belongs to the file delivery
session as specified in this specification MUST set this field
to '2'.
FDT Instance ID, 20 bits:
For each file delivery session, the numbering of FDT Instances starts
from '0' and is incremented by one for each subsequent FDT Instance.
After reaching the maximum value (2^20-1), the numbering starts from
the smallest FDT Instance ID value assigned to an expired FDT
Instance. When wraparound from a greater FDT Instance ID value to a
smaller FDT Instance ID value occurs, the smaller FDT Instance ID
value is considered logically higher than the greater FDT Instance ID
value. Then, the subsequent FDT Instances are assigned the next
available smallest FDT Instance ID value, in order to always keep the
FDT Instance ID values logically increasing.
Senders MUST NOT reuse an FDT Instance ID value that is already in
use for a non-expired FDT Instance. Sender behavior when all the FDT
Instance IDs are used by non-expired FEC Instances is outside the
scope of this specification and left to individual implementations of
FLUTE. Receipt of an FDT Instance that reuses an FDT Instance ID
value that is currently used by a non-expired FDT Instance MUST be
considered an error case. Receiver behavior in this case (e.g.,
leave the session or ignore the new FDT Instance) is outside the
scope of this specification and left to individual implementations of
FLUTE. Receivers MUST be ready to handle FDT Instance ID wraparound
and situations where missing FDT Instance IDs result in increments
larger than one.
Paila, et al. Standards Track [Page 16]

RFC 6726 FLUTE November 20123.4.2. Syntax of FDT Instance
The FDT Instance contains file description entries that provide the
mapping functionality described in Section 3.2 above.
The FDT Instance is an Extensible Markup Language (XML) structure
that has a single root element "FDT-Instance". The "FDT-Instance"
element MUST contain the "Expires" attribute, which provides the
expiration time of the FDT Instance. In addition, the "FDT-Instance"
element MAY contain the "Complete" attribute, a boolean that can be
either set to '1' or 'true' for TRUE, or '0' or 'false' for FALSE.
When TRUE, the "Complete" attribute signals that this "FDT Instance"
includes the set of "File" entries that exhausts both the set of
files delivered so far and the set of files to be delivered in the
session. This implies that no new data will be provided in future
FDT Instances within this session (i.e., that either FDT Instances
with higher ID numbers will not be used or, if they are used, will
only provide file parameters identical to those already given in this
and previous FDT Instances). The "Complete" attribute is therefore
used to provide a complete list of files in an entire FLUTE session
(a "complete FDT"). Note that when all the FDT Instances received so
far have no "Complete" attribute, the receiver MUST consider that the
session is not complete and that new data MAY be provided in future
FDT Instances. This is equivalent to receiving FDT Instances having
the "Complete" attribute set to FALSE.
The "FDT-Instance" element MAY contain attributes that give common
parameters for all files of an FDT Instance. These attributes MAY
also be provided for individual files in the "File" element. Where
the same attribute appears in both the "FDT-Instance" and the "File"
elements, the value of the attribute provided in the "File" element
takes precedence.
For each file to be declared in the given FDT Instance, there is a
single file description entry in the FDT Instance. Each entry is
represented by element "File", which is a child element of the FDT
Instance structure.
The attributes of the "File" element in the XML structure represent
the attributes given to the file that is delivered in the file
delivery session. The value of the XML attribute name corresponds to
the MIME field name, and the XML attribute value corresponds to the
value of the MIME field body [RFC2045]. Each "File" element MUST
contain at least two attributes: "TOI" and "Content-Location". "TOI"
MUST be assigned a valid TOI value as described in Section 3.3.
"Content-Location" [RFC2616] MUST be assigned a syntactically valid
URI, as defined in [RFC3986], which identifies the file to be
delivered. For example, it can be a URI with the "http" or "file"
Paila, et al. Standards Track [Page 17]

RFC 6726 FLUTE November 2012
URI scheme. Only one "Content-Location" attribute is allowed for
each file. The "Content-Location" field MUST be considered a string
that identifies a file (i.e., two different strings are two different
identifiers). Any use of the "Content-Location" field for anything
else other than to identify the object is out of scope of this
specification. The semantics for any two "File" elements declaring
the same "Content-Location" but differing "TOI" is that the element
appearing in the FDT Instance with the greater FDT Instance ID is
considered to declare a newer instance (e.g., version) of the same
"File".
In addition to mandatory attributes, the "FDT-Instance" element and
the "File" element MAY contain other attributes, of which the
following are specifically pointed out:
* The attribute "Content-Type" SHOULD be included and, when present,
MUST be used for the purpose defined in [RFC2616].
* Where the length is described, the attribute "Content-Length" MUST
be used for the purpose defined in [RFC2616]. The transfer length
is defined to be the length of the object transported in octets.
It is often important to convey the transfer length to receivers,
because the source block structure needs to be known for the FEC
decoder to be applied to recover source blocks of the file, and
the transfer length is often needed to properly determine the
source block structure of the file. There generally will be a
difference between the length of the original file and the
transfer length if content encoding is applied to the file before
transport, and thus the "Content-Encoding" attribute is used. If
the file is not content encoded before transport (and thus the
"Content-Encoding" attribute is not used), then the transfer
length is the length of the original file, and in this case the
"Content-Length" is also the transfer length. However, if the
file is content encoded before transport (and thus the
"Content-Encoding" attribute is used), e.g., if compression is
applied before transport to reduce the number of octets that need
to be transferred, then the transfer length is generally different
than the length of the original file, and in this case the
attribute "Transfer-Length" MAY be used to carry the transfer
length.
* Whenever content encoding is applied, the attribute
"Content-Encoding" MUST be included. Whenever the attribute
"Content-Encoding" is included, it MUST be used as described in
[RFC2616].
Paila, et al. Standards Track [Page 18]

RFC 6726 FLUTE November 2012
<xs:attribute name="FEC-OTI-Encoding-Symbol-Length"
type="xs:unsignedLong"
use="optional"/>
<xs:attribute name="FEC-OTI-Max-Number-of-Encoding-Symbols"
type="xs:unsignedLong"
use="optional"/>
<xs:attribute name="FEC-OTI-Scheme-Specific-Info"
type="xs:base64Binary"
use="optional"/>
<xs:anyAttribute processContents="skip"/>
</xs:complexType>
</xs:schema>
END
Figure 3: XML Schema for the FDT Instance
Any valid FDT Instance MUST use the above XML Schema. This way, FDT
provides extensibility to support private elements and private
attributes within the file description entries. Those could be, for
example, the attributes related to the delivery of the file (timing,
packet transmission rate, etc.). Unsupported private elements and
attributes SHOULD be silently ignored by a FLUTE receiver.
In case the basic FDT XML Schema is extended in terms of new
descriptors (attributes or elements), for descriptors applying to a
single file, those MUST be placed within the element "File". For
descriptors applying to all files described by the current FDT
Instance, those MUST be placed within the element "FDT-Instance". It
is RECOMMENDED that the new attributes applied in the FDT be in the
format of message header fields and be either defined in the HTTP/1.1
specification [RFC2616] or another well-known specification, or in an
IANA registry [IANAheaderfields]. However, this specification
doesn't prohibit the use of other formats to allow private attributes
to be used when interoperability is not a concern.
3.4.3. Content Encoding of FDT Instance
The FDT Instance itself MAY be content encoded (e.g., compressed).
This specification defines the FDT Instance Content Encoding Header
(EXT_CENC). EXT_CENC is a new fixed-length LCT Header Extension
[RFC5651]. The Header Extension Type (HET) for the extension is 193.
If the FDT Instance is content encoded, EXT_CENC MUST be used to
signal the content encoding type. In that case, the EXT_CENC Header
Extension MUST be used in all ALC packets carrying the same FDT
Instance ID. Consequently, when the EXT_CENC header is used, it MUST
be used together with a proper FDT Instance Header (EXT_FDT). Within
a file delivery session, FDT Instances that are not content encoded
and FDT Instances that are content encoded MAY both appear. If
Paila, et al. Standards Track [Page 21]

RFC 6726 FLUTE November 2012
content encoding is not used for a given FDT Instance, EXT_CENC MUST
NOT be used in any packet carrying the FDT Instance. The format of
EXT_CENC is defined below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HET = 193 | CENC | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: EXT_CENC Format
Content Encoding Algorithm (CENC), 8 bits:
This field signals the content encoding algorithm used in the FDT
Instance payload. This subsection reserves the Content Encoding
Algorithm values 0, 1, 2, and 3 for null, ZLIB [RFC1950], DEFLATE
[RFC1951], and GZIP [RFC1952], respectively.
Reserved, 16 bits:
This field MUST be set to all '0's. This field MUST be ignored on
reception.
3.5. Multiplexing of Files within a File Delivery Session
The delivered files are carried as transmission objects (identified
with TOIs) in the file delivery session. All these objects,
including the FDT Instances, MAY be multiplexed in any order and in
parallel with each other within a session; i.e., packets for one file
may be interleaved with packets for other files or other FDT
Instances within a session.
Multiple FDT Instances MAY be delivered in a single session using
TOI = 0. In this case, it is RECOMMENDED that the sending of a
previous FDT Instance SHOULD end before the sending of the next FDT
Instance starts. However, due to unexpected network conditions,
packets for the FDT Instances might be interleaved. A receiver can
determine which FDT Instance a packet contains information about,
since the FDT Instances are uniquely identified by their FDT Instance
ID carried in the EXT_FDT headers.
Paila, et al. Standards Track [Page 22]

RFC 6726 FLUTE November 20124. Channels, Congestion Control, and Timing
ALC/LCT has a concept of channels and congestion control. There are
four scenarios in which FLUTE is envisioned to be applied.
(a) Use of a single channel and a single-rate congestion control
protocol.
(b) Use of multiple channels and a multiple-rate congestion control
protocol. In this case, the FDT Instances MAY be delivered on
more than one channel.
(c) Use of a single channel without congestion control supplied by
ALC, but only when in a controlled network environment where
flow/congestion control is being provided by other means.
(d) Use of multiple channels without congestion control supplied by
ALC, but only when in a controlled network environment where
flow/congestion control is being provided by other means. In
this case, the FDT Instances MAY be delivered on more than one
channel.
When using just one channel for a file delivery session, as in (a)
and (c), the notion of 'prior' and 'after' are intuitively defined
for the delivery of objects with respect to their delivery times.
However, if multiple channels are used, as in (b) and (d), it is not
straightforward to state that an object was delivered 'prior' to the
other. An object may begin to be delivered on one or more of those
channels before the delivery of a second object begins. However, the
use of multiple channels/layers may mean that the delivery of the
second object is completed before the first. This is not a problem
when objects are delivered sequentially using a single channel.
Thus, if the application of FLUTE has a mandatory or critical
requirement that the first transmission object must complete 'prior'
to the second one, it is RECOMMENDED that only a single channel be
used for the file delivery session.
Paila, et al. Standards Track [Page 23]

RFC 6726 FLUTE November 2012
Furthermore, if multiple channels are used, then a receiver joined to
the session at a low reception rate will only be joined to the lower
layers of the session. Thus, since the reception of FDT Instances is
of higher priority than the reception of files (because the reception
of files depends on the reception of an FDT Instance describing it),
the following are RECOMMENDED:
1. The layers to which packets for FDT Instances are sent SHOULD NOT
be biased towards those layers to which lower-rate receivers are
not joined. For example, it is okay to put all the packets for
an FDT Instance into the lowest layer (if this layer carries
enough packets to deliver the FDT to higher-rate receivers in a
reasonable amount of time), but it is not okay to put all the
packets for an FDT Instance into the higher layers that only
higher-rate receivers will receive.
2. If FDT Instances are generally longer than one Encoding Symbol in
length and some packets for FDT Instances are sent to layers that
lower-rate receivers do not receive, an FEC encoding other than
Compact No-Code FEC Encoding ID 0 [RFC5445] SHOULD be used to
deliver FDT Instances. This is because in this case, even when
there is no packet loss in the network, a lower-rate receiver
will not receive all packets sent for an FDT Instance.
5. Delivering FEC Object Transmission Information
FLUTE inherits the use of the FEC building block [RFC5052] from ALC.
When using FLUTE for file delivery over ALC, the FEC Object
Transmission Information MUST be delivered in-band within the file
delivery session. There are two methods to achieve this: the use of
the ALC-specific LCT Header Extension EXT_FTI [RFC5775] and the use
of the FDT. The latter method is specified in this section. The use
of EXT_FTI requires repetition of the FEC Object Transmission
Information to ensure reception (though not necessarily in every
packet) and thus may entail higher overhead than the use of the FDT,
but may also provide more timely delivery of the FEC Object
Transmission Information.
The receiver of a file delivery session MUST support delivery of FEC
Object Transmission Information using EXT_FTI for the FDT Instances
carried using TOI value 0. For the TOI values other than 0, the
receiver MUST support both methods: the use of EXT_FTI and the use of
the FDT.
Paila, et al. Standards Track [Page 24]

RFC 6726 FLUTE November 2012
The FEC Object Transmission Information that needs to be delivered to
receivers MUST be exactly the same whether it is delivered using
EXT_FTI or using the FDT (or both). The FEC Object Transmission
Information that MUST be delivered to receivers is defined by the FEC
Scheme. This section describes the delivery using the FDT.
The FEC Object Transmission Information regarding a given TOI may be
available from several sources. In this case, it is RECOMMENDED that
the receiver of the file delivery session prioritize the sources in
the following way (in order of decreasing priority).
1. FEC Object Transmission Information that is available in EXT_FTI.
2. FEC Object Transmission Information that is available in the FDT.
The FDT delivers FEC Object Transmission Information for each file
using an appropriate attribute within the "FDT-Instance" or the
"File" element of the FDT structure.
* "Transfer-Length" carries the "Transfer-Length" Object
Transmission Information element defined in [RFC5052].
* "FEC-OTI-FEC-Encoding-ID" carries the "FEC Encoding ID" Object
Transmission Information element defined in [RFC5052], as carried
in the Codepoint field of the ALC/LCT header.
* "FEC-OTI-FEC-Instance-ID" carries the "FEC Instance ID" Object
Transmission Information element defined in [RFC5052] for
Under-Specified FEC Schemes.
* "FEC-OTI-Maximum-Source-Block-Length" carries the
"Maximum-Source-Block-Length" Object Transmission Information
element defined in [RFC5052], if required by the FEC Scheme.
* "FEC-OTI-Encoding-Symbol-Length" carries the
"Encoding-Symbol-Length" Object Transmission Information element
defined in [RFC5052], if required by the FEC Scheme.
* "FEC-OTI-Max-Number-of-Encoding-Symbols" carries the
"Max-Number-of-Encoding-Symbols" Object Transmission Information
element defined in [RFC5052], if required by the FEC Scheme.
* "FEC-OTI-Scheme-Specific-Info" carries the "encoded
Scheme-specific FEC Object Transmission Information" as defined in
[RFC5052], if required by the FEC Scheme.
Paila, et al. Standards Track [Page 25]

RFC 6726 FLUTE November 2012
In FLUTE, the FEC Encoding ID (8 bits) for a given TOI MUST be
carried in the Codepoint field of the ALC/LCT header. When the FEC
Object Transmission Information for this TOI is delivered through the
FDT, then the associated "FEC-OTI-FEC-Encoding-ID" attribute and the
Codepoint field of all packets for this TOI MUST be the same.
6. Describing File Delivery Sessions
To start receiving a file delivery session, the receiver needs to
know transport parameters associated with the session. Interpreting
these parameters and starting the reception therefore represent the
entry point from which thereafter the receiver operation falls into
the scope of this specification. According to [RFC5775], the
transport parameters of an ALC/LCT session that the receiver needs to
know are:
* The source IP address;
* The number of channels in the session;
* The destination IP address and port number for each channel in the
session;
* The Transport Session Identifier (TSI) of the session;
* An indication that the session is a FLUTE session. The need to
demultiplex objects upon reception is implicit in any use of
FLUTE, and this fulfills the ALC requirement of an indication of
whether or not a session carries packets for more than one object
(all FLUTE sessions carry packets for more than one object).
Optionally, the following parameters MAY be associated with the
session (note that the list is not exhaustive):
* The start time and end time of the session;
* FEC Encoding ID and FEC Instance ID when the default FEC Encoding
ID 0 is not used for the delivery of the FDT;
* Content encoding format if optional content encoding of the FDT
Instance is used, e.g., compression;
* Some information that tells receiver, in the first place, that the
session contains files that are of interest;
* Definition and configuration of a congestion control mechanism for
the session;
Paila, et al. Standards Track [Page 26]

RFC 6726 FLUTE November 2012
* Security parameters relevant for the session;
* FLUTE version number.
It is envisioned that these parameters would be described according
to some session description syntax (such as SDP [RFC4566] or XML
based) and held in a file that would be acquired by the receiver
before the FLUTE session begins by means of some transport protocol
(such as the Session Announcement Protocol (SAP) [RFC2974], email,
HTTP [RFC2616], SIP [RFC3261], manual preconfiguration, etc.).
However, the way in which the receiver discovers the above-mentioned
parameters is out of scope of this document, as it is for LCT and
ALC. In particular, this specification does not mandate or exclude
any mechanism.
7. Security Considerations7.1. Problem Statement
A content delivery system is potentially subject to attacks. Attacks
may target:
* the network (to compromise the routing infrastructure, e.g., by
creating congestion),
* the Content Delivery Protocol (CDP) (e.g., to compromise the
normal behavior of FLUTE), or
* the content itself (e.g., to corrupt the files being transmitted).
These attacks can be launched either:
* against the data flow itself (e.g., by sending forged packets),
* against the session control parameters (e.g., by corrupting the
session description, the FDT Instances, or the ALC/LCT control
parameters) that are sent either in-band or out-of-band, or
* against some associated building blocks (e.g., the congestion
control component).
In the following sections, we provide more details on these possible
attacks and sketch some possible countermeasures. We provide
recommendations in Section 7.5.
Paila, et al. Standards Track [Page 27]

RFC 6726 FLUTE November 20127.2. Attacks against the Data Flow
Let us consider attacks against the data flow first. At the least,
the following types of attacks exist:
* attacks that are meant to give access to a confidential file
(e.g., in the case of non-free content) and
* attacks that try to corrupt the file being transmitted (e.g., to
inject malicious code within a file, or to prevent a receiver from
using a file, which is a kind of denial of service (DoS)).
7.2.1. Access to Confidential Files
Access control to the file being transmitted is typically provided by
means of encryption. This encryption can be done over the whole
file, i.e., before applying FEC protection (e.g., by the content
provider, before submitting the file to FLUTE), or can be done on a
packet-by-packet basis (e.g., when IPsec/ESP [RFC4303] is used; see
Section 7.5). If confidentiality is a concern, it is RECOMMENDED
that one of these solutions be used.
7.2.2. File Corruption
Protection against corruptions (e.g., if an attacker sends forged
packets) is achieved by means of a content integrity verification/
sender authentication scheme. This service can be provided at the
file level, i.e., before applying content encoding and FEC encoding.
In that case, a receiver has no way to identify which symbol(s)
is(are) corrupted if the file is detected as corrupted. This service
can also be provided at the packet level, i.e., after applying
content encoding and FEC encoding, on a packet-by-packet basis. In
this case, after removing all corrupted packets, the file may be in
some cases recovered from the remaining correct packets.
Integrity protection applied at the file level has the advantage of
lower overhead, since only relatively few bits are added to provide
the integrity protection compared to the file size. However, it has
the disadvantage that it cannot distinguish between correct packets
and corrupt packets, and therefore correct packets, which may form
the majority of packets received, may be unusable. Integrity
protection applied at the packet level has the advantage that it can
distinguish between correct and corrupt packets, at the cost of
additional per-packet overhead.
Paila, et al. Standards Track [Page 28]

RFC 6726 FLUTE November 2012
Several techniques can provide this source authentication/content
integrity service:
* At the file level, the file MAY be digitally signed (e.g., by
using RSA Probabilistic Signature Scheme Public-Key Cryptography
Standards version 1.5 (RSASSA-PKCS1-v1_5) [RFC3447]). This
signature enables a receiver to check the file's integrity once
the file has been fully decoded. Even if digital signatures are
computationally expensive, this calculation occurs only once per
file, which is usually acceptable.
* At the packet level, each packet can be digitally signed
[RFC6584]. A major limitation is the high computational and
transmission overheads that this solution requires. To avoid this
problem, the signature may span a set of symbols (instead of a
single one) in order to amortize the signature calculation, but if
a single symbol is missing, the integrity of the whole set cannot
be checked.
* At the packet level, a Group-Keyed Message Authentication Code
(MAC) [RFC2104] [RFC6584] scheme can be used; an example is using
HMAC-SHA-256 with a secret key shared by all the group members,
senders, and receivers. This technique creates a
cryptographically secured digest of a packet that is sent along
with the packet. The Group-Keyed MAC scheme does not create
prohibitive processing load or transmission overhead, but it has a
major limitation: it only provides a group authentication/
integrity service, since all group members share the same secret
group key, which means that each member can send a forged packet.
It is therefore restricted to situations where group members are
fully trusted (or in association with another technique as a
pre-check).
* At the packet level, Timed Efficient Stream Loss-Tolerant
Authentication (TESLA) [RFC4082] [RFC5776] is an attractive
solution that is robust to losses, provides a true authentication/
integrity service, and does not create any prohibitive processing
load or transmission overhead. However, checking a packet
requires a small delay (a second or more) after its reception.
* At the packet level, IPsec/ESP [RFC4303] can be used to check the
integrity and authenticate the sender of all the packets being
exchanged in a session (see Section 7.5).
Techniques relying on public key cryptography (digital signatures and
TESLA during the bootstrap process, when used) require that public
keys be securely associated to the entities. This can be achieved by
Paila, et al. Standards Track [Page 29]

RFC 6726 FLUTE November 2012
a Public Key Infrastructure (PKI), or by a Pretty Good Privacy (PGP)
Web of Trust, or by pre-distributing the public keys of each group
member.
Techniques relying on symmetric key cryptography (Group-Keyed MAC)
require that a secret key be shared by all group members. This can
be achieved by means of a group key management protocol, or simply by
pre-distributing the secret key (but this manual solution has many
limitations).
It is up to the developer and deployer, who know the security
requirements and features of the target application area, to define
which solution is the most appropriate. Nonetheless, in case there
is any concern of the threat of file corruption, it is RECOMMENDED
that at least one of these techniques be used.
7.3. Attacks against the Session Control Parameters and Associated Building Blocks
Let us now consider attacks against the session control parameters
and the associated building blocks. The attacker has at least the
following opportunities to launch an attack:
* the attack can target the session description,
* the attack can target the FDT Instances,
* the attack can target the ALC/LCT parameters, carried within the
LCT header, or
* the attack can target the FLUTE associated building blocks (e.g.,
the multiple-rate congestion control protocol).
The consequences of these attacks are potentially serious, since they
might compromise the behavior of the content delivery system itself.
7.3.1. Attacks against the Session Description
A FLUTE receiver may potentially obtain an incorrect session
description for the session. The consequence of this is that
legitimate receivers with the wrong session description are unable to
correctly receive the session content, or that receivers
inadvertently try to receive at a much higher rate than they are
capable of, thereby possibly disrupting other traffic in the network.
To avoid these problems, it is RECOMMENDED that measures be taken to
prevent receivers from accepting incorrect session descriptions. One
such measure is source authentication to ensure that receivers only
Paila, et al. Standards Track [Page 30]

RFC 6726 FLUTE November 2012
accept legitimate session descriptions from authorized senders. How
these measures are achieved is outside the scope of this document,
since this session description is usually carried out-of-band.
7.3.2. Attacks against the FDT Instances
Corrupting the FDT Instances is one way to create a DoS attack. For
example, the attacker changes the MD5 sum associated to a file. This
possibly leads a receiver to reject the files received, no matter
whether the files have been correctly received or not.
Corrupting the FDT Instances is also a way to make the reception
process more costly than it should be. This can be achieved by
changing the FEC Object Transmission Information when the FEC Object
Transmission Information is included in the FDT Instance. For
example, an attacker may corrupt the FDT Instance in such a way that
Reed-Solomon over GF(2^^16) would be used instead of GF(2^^8) with
FEC Encoding ID 2. This may significantly increase the processing
load while compromising FEC decoding.
More generally, because FDT Instance data is structured using the XML
language by means of an XML media type, many of the security
considerations described in [RFC3023] and [RFC3470] also apply to
such data.
It is therefore RECOMMENDED that measures be taken to guarantee the
integrity and to check the sender's identity of the FDT Instances.
To that purpose, one of the countermeasures mentioned above
(Section 7.2.2) SHOULD be used. These measures will either be
applied on a packet level or globally over the whole FDT Instance
object. Additionally, XML digital signatures [RFC3275] are a way to
protect the FDT Instance by digitally signing it. When there is no
packet-level integrity verification scheme, it is RECOMMENDED to rely
on XML digital signatures of the FDT Instances.
7.3.3. Attacks against the ALC/LCT Parameters
By corrupting the ALC/LCT header (or header extensions), one can
execute attacks on the underlying ALC/LCT implementation. For
example, sending forged ALC packets with the Close Session flag (A)
set to one can lead the receiver to prematurely close the session.
Similarly, sending forged ALC packets with the Close Object flag (B)
set to one can lead the receiver to prematurely give up the reception
of an object.
Paila, et al. Standards Track [Page 31]

RFC 6726 FLUTE November 2012
It is therefore RECOMMENDED that measures be taken to guarantee the
integrity and to check the sender's identity of the ALC packets
received. To that purpose, one of the countermeasures mentioned
above (Section 7.2.2) SHOULD be used.
7.3.4. Attacks against the Associated Building Blocks
Let us first focus on the congestion control building block, which
may be used in the ALC session. A receiver with an incorrect or
corrupted implementation of the multiple-rate congestion control
building block may affect the health of the network in the path
between the sender and the receiver. That may also affect the
reception rates of other receivers who joined the session.
When the congestion control building block is applied with FLUTE, it
is RECOMMENDED that receivers be required to identify themselves as
legitimate before they receive the session description needed to join
the session. How receivers identify themselves as legitimate is
outside the scope of this document. If authenticating a receiver
does not prevent this receiver from launching an attack, this
authentication will enable the network operator to identify him and
to take countermeasures.
When the congestion control building block is applied with FLUTE, it
is also RECOMMENDED that a packet-level authentication scheme be
used, as explained in Section 7.2.2. Some of them, like TESLA, only
provide a delayed authentication service, whereas congestion control
requires a rapid reaction. It is therefore RECOMMENDED [RFC5775]
that a receiver using TESLA quickly reduce its subscription level
when the receiver believes that congestion did occur, even if the
packet has not yet been authenticated. Therefore, TESLA will not
prevent DoS attacks where an attacker makes the receiver believe that
congestion occurred. This is an issue for the receiver, but this
will not compromise the network. Other authentication methods that
do not feature this delayed authentication could be preferred, or a
Group-Keyed MAC scheme could be used in parallel with TESLA to
prevent attacks launched from outside of the group.
7.4. Other Security Considerations
The security considerations that apply to, and are described in, ALC
[RFC5775], LCT [RFC5651], and FEC [RFC5052] also apply to FLUTE, as
FLUTE builds on those specifications. In addition, any security
considerations that apply to any congestion control building block
used in conjunction with FLUTE also apply to FLUTE.
Paila, et al. Standards Track [Page 32]

RFC 6726 FLUTE November 2012
Even if FLUTE defines a purely unidirectional delivery service,
without any feedback information that would be sent to the sender,
security considerations MAY require bidirectional communications.
For instance, if an automated key management scheme is used, a
bidirectional point-to-point channel is often needed to establish a
shared secret between each receiver and the sender. Each shared
secret can then be used to distribute additional keys to the
associated receiver (e.g., traffic encryption keys).
As an example, [MBMSsecurity] details a complete security framework
for the Third Generation Partnership Project (3GPP) Multimedia
Broadcast/Multicast Service (MBMS) that relies on FLUTE/ALC for
Download Sessions. It relies on bidirectional point-to-point
communications for User Equipment authentication and for key
distribution, using the Multimedia Internet KEYing (MIKEY) protocol
[RFC3830]. Because this security framework is specific to this use
case, it cannot be reused as such for generic security
recommendations in this specification. Instead, the following
section introduces minimum security recommendations.
7.5. Minimum Security Recommendations
We now introduce a mandatory-to-implement, but not necessarily to
use, security configuration, in the sense of [RFC3365]. Since FLUTE
relies on ALC/LCT, it inherits the "baseline secure ALC operation" of
[RFC5775]. More precisely, security is achieved by means of IPsec/
ESP in transport mode. [RFC4303] explains that ESP can be used to
potentially provide confidentiality, data origin authentication,
content integrity, anti-replay, and (limited) traffic flow
confidentiality. [RFC5775] specifies that the data origin
authentication, content integrity, and anti-replay services SHALL be
supported, and that the confidentiality service is RECOMMENDED. If a
short-lived session MAY rely on manual keying, it is also RECOMMENDED
that an automated key management scheme be used, especially in the
case of long-lived sessions.
Therefore, the RECOMMENDED solution for FLUTE provides per-packet
security, with data origin authentication, integrity verification,
and anti-replay. This is sufficient to prevent most of the in-band
attacks listed above. If confidentiality is required, a per-packet
encryption SHOULD also be used.
Paila, et al. Standards Track [Page 33]

RFC 6726 FLUTE November 20128.3. Registration of the application/fdt+xml Media Type
IANA has registered the following in the "Application Media Types"
registry at http://www.iana.org/assignments/media-types/application/.
Type name: application
Subtype name: fdt+xml
Required parameters: none
Optional parameters: charset="utf-8"
Encoding considerations: binary (the FLUTE file delivery protocol
does not impose any restriction on the objects it carries and in
particular on the FDT Instance itself)
Restrictions on usage: none
Security considerations: fdt+xml data is passive and does not
generally represent a unique or new security threat. However, there
is some risk in sharing any kind of data, in that unintentional
information may be exposed, and that risk applies to fdt+xml data as
well.
Interoperability considerations: None
Published specification: [RFC6726], especially noting Section 3.4.2.
The specified FDT Instance functions as an actual media format of use
to the general Internet community, and thus media type registration
under the Standards Tree is appropriate to maximize interoperability.
Applications that use this media type: file and object delivery
applications and protocols (e.g., FLUTE).
Additional information:
Magic number(s): none
File extension(s): ".fdt" (e.g., if there is a need to store an
FDT Instance as a file)
Macintosh File Type Code(s): none
Person and email address to contact for further information:
Toni Paila (toni.paila@gmail.com)
Intended usage: Common
Author/Change controller: IETF
Paila, et al. Standards Track [Page 35]

RFC 6726 FLUTE November 201210. Contributors
Jani Peltotalo
Tampere University of Technology
P.O. Box 553 (Korkeakoulunkatu 1)
Tampere FIN-33101
Finland
EMail: jani.peltotalo@tut.fi
Sami Peltotalo
Tampere University of Technology
P.O. Box 553 (Korkeakoulunkatu 1)
Tampere FIN-33101
Finland
EMail: sami.peltotalo@tut.fi
Magnus Westerlund
Ericsson Research
Ericsson AB
SE-164 80 Stockholm
Sweden
EMail: magnus.westerlund@ericsson.com
Thorsten Lohmar
Ericsson Research (EDD)
Ericsson Allee 1
52134 Herzogenrath
Germany
EMail: thorsten.lohmar@ericsson.com
11. Change Log11.1. RFC 3926 to This Document
Incremented the FLUTE protocol version from 1 to 2, due to concerns
about backwards compatibility. For instance, the LCT header changed
between RFC 3451 and [RFC5651]. In RFC 3451, the T and R fields of
the LCT header indicate the presence of Sender Current Time and
Expected Residual Time, respectively. In [RFC5651], these fields
MUST be set to zero and MUST be ignored by receivers (instead, the
EXT_TIME Header Extensions can convey this information if needed).
Thus, [RFC5651] is not backwards compatible with RFC 3451, even
though both use LCT version 1. FLUTE version 1 as specified in
[RFC3926] MUST use RFC 3451. FLUTE version 2 as specified in this
document MUST use [RFC5651]. Therefore, an implementation that
relies on [RFC3926] and RFC 3451 will not be backwards compatible
with FLUTE as specified in this document.
Paila, et al. Standards Track [Page 37]

RFC 6726 FLUTE November 2012
Updated dependencies to other RFCs to revised versions; e.g., changed
ALC reference from RFC 3450 to [RFC5775], changed LCT reference from
RFC 3451 to [RFC5651], etc.
Added clarification for the use of FLUTE for unicast communications
in Section 1.1.4.
Clarified how to reliably deliver the FDT in Section 3.3 and the
possibility of using out-of-band delivery of FDT information.
Clarified how to address FDT Instance expiration time wraparound with
the notion of the NTPv4 "epoch" in Section 3.3.
Clarified what should be considered erroneous situations in
Section 3.4.1 (definition of FDT Instance ID). In particular, a
receiver MUST be ready to handle FDT Instance ID wraparounds and
missing FDT Instances.
Updated Section 7.5 to define IPsec/ESP as a mandatory-to-implement
security solution.
Removed the 'Statement of Intent' from Section 1. The statement of
intent was meant to clarify the "Experimental" status of [RFC3926].
It does not apply to this document.
Added clarification of "XML-DSIG" near the end of Section 3.
In Section 3.2, replaced "complete FDT" with text that is more
descriptive.
Clarified Figure 1 with regard to "Encoding Symbol(s) for FDT
Instance".
Clarified the text regarding FDT Instance ID wraparound at the end of
Section 3.4.1.
Clarified "complete FDT" in Section 3.4.2.
Added semantics for the case where two TOIs refer to the same
Content-Location. It is now in line with the way that 3GPP and
Digital Video Broadcasting (DVB) standards interpret this case.
In Section 3.4.2, the XML Schema of the FDT Instance was modified per
advice from various sources. For example, extension by element was
missing but is now supported. Also, the namespace definition was
changed to URN format.
Clarified FDT-schema extensibility at the end of Section 3.4.2.
Paila, et al. Standards Track [Page 38]

RFC 6726 FLUTE November 2012
The CENC value allocation has been added at the end of Section 3.4.3.
Section 5 has been modified so that EXT_FTI and the FEC issues were
replaced by a reference to the ALC specification [RFC5775].
Added a clarifying paragraph on the use of the Codepoint field at the
end of Section 5.
Reworked Section 8 -- IANA Considerations; it now contains six IANA
registration requests:
* Registration of the FDT Instance XML Namespace.
* Registration of the FDT Instance XML Schema.
* Registration of the application/fdt+xml Media Type.
* Registration of the Content Encoding Algorithms.
* Registration of two LCT Header Extension Types and corresponding
values in the LCT Header Extension Types Registry (192 for EXT_FDT
and 193 for EXT_CENC).
Added Section 10 -- Contributors.
Revised lists of both Normative and Informative references.
Added a clarification that the receiver should ignore reserved bits
of Header Extension type 193 upon reception.
Elaborated on what kinds of networks cannot support FLUTE congestion
control (Section 1.1.4).
In Section 3.2, changed "several" (meaning 3-n vs. "couple" = 2) to
"multiple" (meaning 2-n).
Moved the requirement in Section 3.3 (to send FDT more reliably than
files) to a bulleted RECOMMENDED requirement, making check-off easier
for testers.
In Section 3.3, sharpened the definition that future FDT file
instances can "augment" (meaning enhance) rather than "complement"
(sometimes meaning negate, which is not allowed) the file parameters.
Elaborated in Sections 3.3 and 4 that FEC Encoding ID = 0 is Compact
No-Code FEC, so that the reader doesn't have to search other RFCs to
understand these protocol constants used by FLUTE.
Paila, et al. Standards Track [Page 39]

RFC 6726 FLUTE November 2012Appendix A. Receiver Operation (Informative)
This section gives an example of how the receiver of the file
delivery session may operate. Instead of a detailed state-by-state
specification, the following should be interpreted as a rough
sequence of an envisioned file delivery receiver.
1. The receiver obtains the description of the file delivery session
identified by the (source IP address, Transport Session
Identifier) pair. The receiver also obtains the destination IP
addresses and respective ports associated with the file delivery
session.
2. The receiver joins the channels in order to receive packets
associated with the file delivery session. The receiver may
schedule this join operation utilizing the timing information
contained in a possible description of the file delivery session.
3. The receiver receives ALC/LCT packets associated with the file
delivery session. The receiver checks that the packets match the
declared Transport Session Identifier. If not, the packets are
silently discarded.
4. While receiving, the receiver demultiplexes packets based on
their TOI and stores the relevant packet information in an
appropriate area for recovery of the corresponding file.
Multiple files can be reconstructed concurrently.
5. The receiver recovers an object. An object can be recovered when
an appropriate set of packets containing Encoding Symbols for the
transmission object has been received. An appropriate set of
packets is dependent on the properties of the FEC Encoding ID and
FEC Instance ID, and on other information contained in the FEC
Object Transmission Information.
6. Objects with TOI = 0 are reserved for FDT Instances. All FDT
Instances are signaled by including an EXT_FDT Header Extension
in the LCT header. The EXT_FDT header contains an FDT Instance
ID (i.e., an FDT version number). If the object has an FDT
Instance ID 'N', the receiver parses the payload of the instance
'N' of the FDT and updates its FDT database accordingly.
7. If the object recovered is not an FDT Instance but a file, the
receiver looks up its FDT database to get the properties
described in the database, and assigns the file the given
properties. The receiver also checks that the received content
Paila, et al. Standards Track [Page 44]

RFC 6726 FLUTE November 2012
length matches with the description in the database. Optionally,
if an MD5 checksum has been used, the receiver checks that the
calculated MD5 matches the description in the FDT database.
8. The actions the receiver takes with imperfectly received files
(missing data, mismatching content integrity digest, etc.) are
outside the scope of this specification. When a file is
recovered before the associated file description entry is
available, a possible behavior is to wait until an FDT Instance
is received that includes the missing properties.
9. If the file delivery session end time has not been reached, go
back to step 3. Otherwise, end.
Appendix B. Example of FDT Instance (Informative)
<?xml version="1.0" encoding="UTF-8"?>
<FDT-Instance xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="urn:ietf:params:xml:ns:fdt
ietf-flute-fdt.xsd"
Expires="2890842807">
<File
Content-Location="http://www.example.com/menu/tracklist.html"
TOI="1"
Content-Type="text/html"/>
<File
Content-Location="http://www.example.com/tracks/track1.mp3"
TOI="2"
Content-Length="6100"
Content-Type="audio/mp3"
Content-Encoding="gzip"
Content-MD5="+VP5IrWploFkZWc11iLDdA=="
Some-Private-Extension-Tag="abc123"/>
</FDT-Instance>
Paila, et al. Standards Track [Page 45]